JPA Tutorial 4 - Introduction

So far, every query we've used has returned a single kind of result. In this tutorial we change things up a bit by moving from only using Books in our Library to working with Books and Dvd's, both of which inherit from Resource. When we search for a Resource we might get back Books, Dvd's, or both. This makes our queries polymorphic.

We pick up at the end of JPA Tutorial 3 - A Mini Application and perform two major transformations:

1. Introduce the Resource class and make the appropriate refactorings
2. Introduce a new type, Dvd